Electropolishing is a well-known process for industrial large scale finishing of conducting surfaces. Electropolishing is most commonly used in static set-ups where the parts to be polished are immersed in a polishing bath. Less common, but still widespread in industry, is the use of electropolishing in a continuous process.
In electropolishing, a positive electrical potential is applied to a conducting material which is to be polished. The material is placed into an acid bath where there are additional electrodes to complete the electrical circuit. The built up electrical field strength depends locally on the surface topology. Protrusions on the material develop a stronger electrical field than grooves and holes. The etching of the material by the acid depends on the local current density at the acid-surface interface. The current density is directly related to the electrical field strength (Ohm's law), therefore, etch rates depend on the surface topology. Etching is enhanced on protrusions and weakened on grooves and holes. This results in a smoothening of the surface if suitable operating parameters are used. The process can be influenced by the following parameters: applied voltage/current, type of acid, acid concentration, temperature, and polishing time.
Coated conductors (superconductive tapes or films) usually involve a thick superconductive film on a polycrystalline metallic substrate wherein there are a number of intermediate layers between the superconductive film and the metallic substrate. These intermediate layers generally have a number of roles including, e.g., providing better smoothness to the base substrate, providing nucleation sites for subsequent layers, providing an epitaxial structure for alignment of subsequent layers, and providing buffer layers with structural and/or chemical compatibility to both other layers and any deposited superconductive film.
When the initial base substrate for a coated conductor is a polycrystalline nickel alloy, it usually has significant roughness, e.g., a root mean square (RMS) roughness of from 15 nanometers (nm) to 100 nm or greater. Such roughness hinders the subsequent deposition of optimal coating layers which results in reduced current carrying capacity by the subsequent superconductive film. Accordingly, there have been numerous efforts to reduce the roughness. Generally, to obtain the desired smoothness, the metallic substrate has been polished, either mechanically or chemically mechanically, to provide a smoother surface. Drawbacks to these approaches include processing speeds and the difficulty of scale-up. Enhanced smoothness for subsequent superconductive film depositions has also been sought through another coating layer, e.g., an inert oxide material layer upon the metallic substrate, where that layer can also be treated by chemical mechanical polishing. Drawbacks to this approach include the need for the extra layer of inert oxide material as well as processing speeds.
Further improvements in the structure and resultant properties of coated conductors remain desirable. After extensive and careful investigation, improvements have now been found in the preparation of highly smooth base metallic substrates having a RMS roughness of less than about 2 nm, preferably less than about 1 nm. Such highly smooth base metallic substrates can be used in obtaining high quality structural template articles desirable for the subsequent deposition of oriented films, e.g., high temperature superconducting films, especially YBCO superconducting films. Other applications for such highly smooth base metallic substrates may also be recognized by one skilled in the art.
It is an object of the present invention to provide a high current density electropolishing process for the preparation of highly smooth metallic substrates, especially metallic tapes.
It is another object of the present invention to provide a high current density electropolishing process for the preparation of highly smooth polycrystalline metal substrate tapes for coated conductor production.